Cross-linked, cationic, polymeric compositions are known to those skilled in the art and are useful, after having been subjected to high shearing to render them soluble, in a variety of solid-liquid separation applications, particularly in the flocculation of various dispersions of suspended solids, such as sewage sludge, and in the thickening of cellulosic paper pulp suspensions. Modern concerns about environmental pollution and the increasing cost of materials useful therein have made it highly desirable to produce flocculating agents winch cause higher degrees of separation at lower dosage levels.
EP 0,202,780 describes the preparation of crosslinked, cationic, polyacrylamide beads by conventional inverse emulsion polymerization techniques. Crosslinking is accomplished by the incorporation of a difunctional monomer, such as methylenebisacrylamide, into the polymer chain. This cross-linking technology is well known in the art. The patentee teaches that the crosslinked beads are useful as flocculants but are more highly efficient after having been subjected to unusual levels of shearing action in order to render them water-soluble.
Typically, the particle size of polymers prepared by conventional, inverse, water-in-oil, emulsion, polymerization processes are limited to a range of 1-5 microns, since no particular advantage in reducing the particle size has hitherto been apparent. The particle size which is achievable in inverse emulsions is determined by the concentration and activity of the surfactant(s) employed and these are customarily chosen on the basis of emulsion stability and economic factors.
Leong, et al., in Inverse Microemulsion Polymerization, J. of Phys. Chem., Vol. 86, No. 23, Jun. 24, 1982, pp 2271-3, discloses polymerization of acrylamide in an inverse microemulsion. The authors also disclose having prepared a cross-linked polyacrylamide latex or microgel by using a 100:1 mixture of acrylamide-methylenebisacrylamide. No use of a cationic monomer is mentioned or of the possible use of the resultant product as a flocculating agent.
The present invention describes the preparation of a novel class of cationic, cross-linked, polymeric, microparticle flocculants. The new polymers are prepared by the optimal use of a variety of high activity surfactant or surfactant mixtures to achieve submicroparticle size. Preferably, the type and concentration of surfactant should be chosen to yield a particle size of less than about 0.5 micron in diameter and more preferably less than about 0.1 micron. Unexpectedly, the novel polymers of this invention are useful in flocculating applications without the necessity of being subjected to high and unusual levels of shearing action and surprisingly show improved performance over the prior art cationic flocculants prepared by conventional inverse emulsion polymerization. The products of this invention, when used as flocculating agents, cause high degrees of separation at low dosage levels thus satisfying both environmental and economical concerns and therefore constituting a significant advancement in the state of the art.